WASHINGTON — Joe Biden wants to be president. And each day, he's closer to being ready to run for the office. But even as he weighs a campaign to unseat President Donald Trump, Biden is carefully considering a key question — what happens when the president or his top allies try to make his family an issue?

Conversations with aides to the former vice president and others who've spoken with him in recent weeks present the idea of a Biden candidacy as not if but when. Since the start of the year it's been like a "slow boil," as one aide put it, with Biden's answer on whether to run moving in a steady direction toward yes, incrementally warmer with each passing day.

Elected Democrats and key party figures who've spoken with Biden, in person or by phone, say he has been giving percentages of the likelihood of him running — from 70, to 80 and even more recently 90 percent. He speaks regularly with President Obama, who is acting as a sounding board to his former running mate. And he's even called several of his would-be opponents in the 2020 Democratic primary, congratulating them on their announcements and wishing them well even as he may soon face off against them.

Sources close to the former vice president say he's clear-eyed about the political challenge ahead if he runs, not taking anything for granted in a crowded race for the nomination even as he's confident he offers the party the best chance to be Trump.

But Biden knows and expects the president to fight as hard to stay in the White House as he did to win it in the first place — and that he's already shown nothing is off limits.

"I understand how anyone running could be concerned about the ways in which President Trump demonstrated in the 2016 election an enthusiasm for attacking not just his opponents but his family, including famously by making things up," said Sen. Chris Coons, D-Del., who met with Biden recently to discuss the campaign.

"I offered my general advice that he should do what he does best, which is focus on the positive and let others take up the mantle of defending his family, his record and his honor to the extent that's at all necessary," Coons told NBC News.

No line of attack would be more reprehensible to the former vice president than one directed at his family and he and his team have been forced to consider that even as they also weigh the political dynamics.

Biden is the senior statesman and a largely popular figure in a Democratic Party eager to unite quickly behind someone who can win back the White House. But he's also a grandfather and a father trying to keep the family together.

Biden told his closest aides this week he has a few final "traps" to run before a final decision — including gut-check conversations with his children and grandchildren.

"Family is the beginning, the middle and the end. Everything revolves around family," Biden said at one of his few public events in recent weeks.

The initial shock over the passing of his eldest son, Beau, four years ago has passed. But the grieving process never ends — it just takes new forms, as Biden knows better than most. And weeks after the family marked what would have been Beau's 50th birthday, how his absence continues to weigh on the family is something Biden still is mindful of as the family patriarch.

Discussions among his inner circle have included simulating what would happen on the campaign trail if Biden were asked to respond to a fresh Trump tweet or public comment directed at Biden's family.

The trials Biden and his family have endured will undoubtedly be part of the discussion around his candidacy regardless of what the president does. His wife, Jill, has written a book due out in May that its publisher promises will be a "brave and vulnerable glimpse into the creation of a beloved American family."

Biden's second son, Hunter, has also acknowledged the complications of his personal life recently — a difficult divorce and battles with substance abuse. In a recent statement to Vanity Fair, he insisted they wouldn't affect his father's 2020 planning.

"The priority has always been clear for my dad, as it is, now, for me: Never run from a struggle," he said. "I can tell you that I wouldn't be alive today, if my dad hadn't kept fighting for me, too. So the idea that tragedy or tough times or any number of trials would dissuade a Biden from serving his fellow man— whether a friend or a fellow citizen — could not be more misguided."

Biden, indeed, emerged from the holidays more convinced the family is ready for a campaign. They have taken great comfort in returning to their annual rituals, he said at a recent event discussing his 2017 book, "Promise Me, Dad."

That book's title is drawn from the conversation he had with Beau in his final months, when his son urged him not to withdraw from fighting for the causes that have animated him during his decades in public life. Remembering his son's commitment to duty is something Biden said brings him solace and a sense of purpose

"I get up every morning hoping that Beau's proud of me," Biden said at the event, adding later: "I want … Beau's son and daughter to know I was true to their dad's wishes."

When Biden passed on the 2016 race he did so understanding it may have been his last chance to seek the presidency. But Hillary Clinton's shocking defeat to Trump affords the 76-year-old a final unexpected opportunity, at a time when he feels his skills experience are needed perhaps more than ever.

Still, he's also facing the ticking clock of a primary race already underway, a dwindling supply of talent needed to staff a winning campaign, and the already-bubbling impatience of rival campaigns and would-be candidates looking for a definitive answer from the former vice president so they can make their own.

"I think that we have a tendency particularly in the States to start the whole election process much too early," he said during an appearance at the Munich Security Conference. "I think we should be focusing now on what needs to be done to alter some of the policies that are being promoted by the president."

Biden's team has long been working to lay the groundwork for a campaign for months, with mid-March as something of a moving target. Once given a green-light, it would likely take another week or two before any public launch — realistically putting the start into the second fundraising quarter after April 1.

Biden's public schedule has largely been oriented toward the higher education institutions where he has rooted his life since leaving the White House — an event at the Penn Biden Center at the University of Pennsylvania earlier this week, and one at the newly-renamed Joseph R. Biden, Jr. School of Public Policy & Administration at the University of Delaware next Tuesday.

He will also appear at the University of Nebraska at Omaha next week — just across the river from Iowa. Those events are somewhat controlled environments where Biden can focus on the issues he sees as setting him apart in a crowded field — foreign policy, and his appeal to middle class voters.

"I don't begrudge anybody making a million or hundreds of millions of dollars. I really don't," Biden said at Tuesday's event in Philadelphia. "But I do think there's some shared responsibility and it's not being shared fairly for hard-working, middle class and working class people."

As they bide time until a decision, his political team distributed talking points to a small network of outside allies makes the case, arguing the country is looking for a "trusted leader" who "provides a sense of unity for a country that is desperate for a sense of stability."

Several of those points invoke a unique bond Biden has with the public in part because of his own family's story.

"You only have to spend a few minutes with Biden out on the campaign trail to see the ferocity with which people believe in him. They hug him, they implore him to run, they tell him their family's personal struggle. There is a passion for him that is unique," one reads.